ai_18 released

adjusted ice world graphics, added coasts, icebergs.
added further ice world hills.
adjusted farm enclosure graphics.
added planet atmospheres. Added graphics for atmosphere of temp, lava, jungle, desert, and ice worlds.
added planet star field.
added code to twinkle star fields (not yet properly adapted in galactic map, only twinkles if scrolling)
added moon world objects: nebula and Gas giant on horizon.